EP3494003B1 - Method for operating a clutch of a motor vehicle, and motor vehicle - Google Patents

Description

The invention relates to a method for operating a clutch of a motor vehicle according to the preamble of patent claim 1 and to a motor vehicle with a drive train.

Such methods for operating clutches of motor vehicles are already sufficiently known from the general state of the art and in particular from series vehicle construction. The respective clutch is part of a respective drive train, by means of which the respective motor vehicle can be driven. The drive train thus comprises the clutch and a drive motor, which can be designed as an internal combustion engine, for example. In addition, the drive train comprises a primary axle, which is a first axle or is also referred to as a first axle and has first wheels. The first wheels can be driven by the drive motor.

The drive train further comprises a secondary axis which is spaced apart from the primary axis in the vehicle longitudinal direction and which has second wheels. The secondary axle is also referred to as the second axle or is a second axle of the drive train, with the second wheels being drivable by the drive motor via the clutch. For example, the primary axle is a front axle so that the first wheels are front wheels. The secondary axle is, for example, a rear axle arranged behind the front axle in the longitudinal direction of the vehicle, so that the second wheels are, for example, rear wheels.

In the respective method, the respective clutch is adjusted between a closed position and at least one second position different from the closed position. In the closed position, the axles are coupled to one another via the coupling. In other words, the clutch couples the axes in the closed position of the coupling with each other. A first clutch torque of the clutch is set in the closed position. In the second position, which is different from the closed position, the coupling couples the axles less strongly than in the closed position. In this case, a second clutch torque of the clutch that is lower than the first clutch torque is set in the second position. In order to move the clutch from the closed position into the second position, for example, the clutch is at least partially opened. In the closed position, the clutch can transmit a maximum of a first torque, for example, so that a maximum or at most the first torque can be transmitted between the axles via the clutch. In the second position, which is different from the closed position, the clutch can, for example, transmit a maximum of a second torque that is lower than the first torque, so that in the second position a maximum or at most the second torque can be transmitted between the axles via the clutch. Thus, a lower clutch torque of the clutch is set in the second position compared to the closed position.

The US 2005/0121247 A1 discloses a method for operating a drive train of a motor vehicle, in which a clutch, the connecting force of which is variable, is provided between a front axle and a rear axle. As part of the method, the motor vehicle can be operated in a conventional mode and in an anti-vibration mode.

The DE 10 2010 047 443 A1 discloses a vehicle with an all-wheel drive system in which the front axle and the rear axle of the vehicle can be driven with front axle and rear axle torques of different magnitudes, with a torque difference between the front axle and rear axle torques resulting in a speed difference between a front axle speed and a rear axle speed. It is also provided that the vehicle has a unit for determining a road coefficient of friction, the unit detecting the speed difference. The unit determines the road coefficient of friction on the basis of a pair of values consisting of the speed difference and the torque difference or parameters correlating therewith.

In addition, from the DE 10 2009 009 264 A1 a power transmission device for distributing a driving force outputted from a transmission system to a main drive gear and a sub-drive gear is known. The object of the present invention is to further develop a method and a motor vehicle of the type mentioned at the beginning in such a way that particularly comfortable operation of the motor vehicle can be achieved.

Furthermore, the DE 198 38 169 A1 a system for controlling the power distribution of a four-wheel drive vehicle.

In addition, the U.S. 4,669,569 A a four-wheel drive vehicle with an engine known as known.

This object is achieved by a method with the features of claim 1 and by a motor vehicle with the features of claim 8. Advantageous configurations with expedient developments of the invention are specified in the remaining claims.

A first aspect of the invention relates to a method for operating a clutch of a motor vehicle, in particular a motor vehicle such as a passenger car. The clutch is part of a drive train of the motor vehicle, which can be driven by means of the drive train. The drive train thus comprises the clutch and a drive motor, which can be designed, for example, as an internal combustion engine or as an electric machine or as an electric motor.

The drive train comprises a primary axle which has first wheels that can be driven by the drive motor. The primary axis is a first axis of the drive train or is also referred to as the first axis. The drive train further comprises a secondary axis which is spaced apart from the primary axis in the vehicle longitudinal direction and which has second wheels. The secondary axis is a second axis of the drive train or is also referred to as the second axis. The second wheels can be driven by the drive motor via the clutch.

For example, the primary axle is a front axle so that the first wheels are front wheels. The secondary axle is, for example, a rear axle arranged in the longitudinal direction of the vehicle behind the front axle, so that the second wheels are, for example, rear wheels. Thus, for example, the rear axle is designed as a so-called hang-on rear axle. Alternatively, it is conceivable that the rear axle is the primary axle and the front axle is the secondary axle, so that the front axle is then designed as a so-called hang-on front axle.

In the method, the clutch is adjusted between a closed position and at least one second position different from the closed position. In the closed position, the axles are coupled to one another via the coupling. A first clutch torque of the clutch is set in the closed position. In the second position, the coupling couples the axles less strongly than in the closed position. In this case, a second clutch torque of the clutch that is lower than the first clutch torque is set in the second position. For example, in order to move the clutch from the closed position into the second position, the clutch is at least partially opened. This means, for example, that in the closed position of the clutch the axles are coupled to one another more strongly than in the second position of the clutch, so that the axles are less strongly coupled to one another via the clutch in the second position of the clutch than in the closed position of the clutch . For example, in the closed position, the clutch can transmit a maximum or at most a first torque, so that, for example, in the closed position of the clutch, a maximum or at most the first torque can be transmitted between the axles via the clutch. In the second position, the clutch can transmit at most or at most a second torque that is lower than the first torque, so that, for example, in the second position of the clutch, at most or at most the second torque, which is lower than the first torque, can be transmitted between the axles via the clutch . Thus, a lower clutch torque of the clutch is set in the second position compared to the first position.

In order to be able to realize particularly comfortable operation of the drive train and thus of the motor vehicle as a whole, the method according to the invention comprises a first step in which at least one coefficient of friction of a roadway on which the motor vehicle is located is determined. The coefficient of friction is determined, for example, by means of an electronic computing device, in particular of the motor vehicle, the electronic computing device also being referred to as a control unit.

According to the invention, the method further comprises a second step in which, depending on the determined coefficient of friction, in particular by means of the electronic computing device, a basic torque of the clutch is set, which is pretensioned in the second position by the basic torque. This means that the clutch is not completely open in the second position, but rather the clutch is closed in the second position, although the clutch is closed less far or less strongly in the second position than in the closed position. As a result, the axes are coupled to one another via the coupling both in the closed position and in the second position of the coupling, the axes being less strongly coupled to one another in the second position than in the closed position. Thus, for example, in the closed position, at most the first torque that is different from zero and greater than the second torque can be transmitted to the axle via the clutch. In the second position, at most the second torque, which differs from zero and is lower than the first torque, can be transmitted via the coupling between the axles. Thus, for example, in the closed position, a first clutch torque of the clutch that is different from zero is set, wherein in the second position a second clutch torque that is different from zero and is lower than the first clutch torque is set. Here, for example, the second clutch torque is the basic torque which is different from zero and less than the first clutch torque which is set in the closed position of the clutch.

Due to the base torque, which is different from zero, the clutch is pre-tensioned in the second position so that, for example, the clutch can be closed particularly quickly, in particular further closed, starting from the second position, so that the clutch, for example, moves particularly quickly into the closed position starting from the second position can be.

The invention is based on the idea of not using a constant basic torque for pretensioning the clutch in the second position, but rather, according to the invention, the basic torque is varied as a function of the determined coefficient of friction. This can, for example, vibrations or Vibrations of the drive train, in particular during turning maneuvers and thus when cornering the motor vehicle, are avoided or at least kept particularly low, so that a particularly high level of driving comfort, in particular for passengers of the motor vehicle, can be achieved.

The drive train is designed as an all-wheel drive train or as an all-wheel drive system, it being possible to switch between two-wheel drive and four-wheel or all-wheel drive as required by means of the clutch. If the primary axle is the front axle, the two-wheel drive is a front-wheel drive. If the primary axle is the rear axle, the two-wheel drive is a rear-wheel or rear-wheel drive.

To set the four-wheel or all-wheel drive, the clutch is, for example, closed or moved into its closed position so that both the first wheels and the second wheels can be or are driven by the drive motor, in particular when the drive motor is in pulling mode. To implement the two-wheel drive, the clutch is moved, for example, starting from the closed position into the second position and thus opened, the clutch preferably not being completely opened, but being pretensioned in the second position by the basic torque. As a result, for example, with respect to the wheels, only the first wheels are driven by the drive motor or the second wheels are driven less strongly than when the four-wheel or all-wheel drive is set via the clutch, in particular when the drive motor is in pulling mode. Since the basic torque differs from zero, the axes are coupled to one another in the second position of the coupling, for example via the coupling, but the axes are less strongly coupled to one another via the coupling than in the closed position.

It has been found that in clutch-based all-wheel drive systems, in particular during turning maneuvers and thus when cornering, there is a strong tendency to oscillate when the rotational speed compensation of the axes moving, for example, on different path radii. Furthermore, there may be a conflict of objectives between the implementation of advantageous acoustics and the implementation of advantageous traction, since a strong coupling of the axles to be achieved by means of the clutch is desirable in order to implement advantageous traction. To realize the strong When the axles are coupled, the drive train is, for example, pressed over or operated in an overpressed state in which the coupling is so strongly closed in its closed position, in particular is compressed, that no differential speeds or no slip occur or occur in the coupling and between the axles.

In order to achieve advantageous acoustics, however, only a slight coupling to be effected by means of the coupling is desirable, in particular during turning maneuvers in low speed ranges, since there is a particularly strong tendency to vibrate.

In order to defuse or resolve this conflict of objectives, the basic torque is set as a function of the determined coefficient of friction, whereby a situation-adaptive torque cap can be implemented. It is therefore possible, for example, depending on the driving situation, in particular depending on the coefficient of friction, to achieve a strong coupling of the axles via the clutch and thus particularly good traction or only a low coupling of the axles and thus particularly advantageous acoustics of the drive train. The method according to the invention is more advantageous compared to such torque caps or torque reductions that are based on stationary steering angle-dependent maximum torques that are deactivated, for example, during kickdown. It has been found that torque reductions based on steady-state steering angle-dependent maximum torques can lead to unacceptable traction properties, in particular during turning maneuvers, due to low basic torques. This disadvantage or this problem can be avoided by means of the method according to the invention. Using the method according to the invention, it is thus possible to ensure all-wheel or four-wheel properties and thus advantageous traction even in an efficiency-optimized and thus space-saving and weight-saving drive train, while at the same time avoiding excessive vibrations and the resulting excessive impairment of comfort.

The coefficient of friction of the road is determined, in particular estimated, on the basis of a stability analysis of at least one of the wheels, for example. If, for example, in the context of the stability analysis, it is detected that the at least one wheel is at its frictional connection limit at a point in time, which is characterized, for example, by Kamm's circle can be illustrated, comes or exceeds the frictional connection limit, so at the point in time accelerations acting on the motor vehicle, in particular the longitudinal acceleration and the transverse acceleration, are detected. The coefficient of friction can be deduced on the basis of the accelerations, in particular through the addition of force vectors of the longitudinal and transverse acceleration. For example, the sum of the vector addition is the radius of Kamm's circle and a measure of the road surface's coefficient of friction. For example, on the basis of a difference between the speed of the at least one wheel and the speed of at least one other of the wheels, it can be detected that the at least one wheel is approaching or exceeding its frictional connection limit.

According to the invention, an expected behavior, in particular driving behavior, of the motor vehicle is calculated by means of an electronic computing device of the motor vehicle using at least one computation model, the coefficient of friction being determined as a function of the calculated behavior. As a result, the basic torque can be adapted particularly well to the respective driving situations, so that the aforementioned conflict of objectives can be relaxed or resolved.

In order to be able to achieve a particularly high level of driving comfort, it is provided in one embodiment of the invention that the basic torque of the clutch is set as a function of a steering angle of the motor vehicle. The wheels of at least one of the axles, in particular the front wheels, are designed, for example, as steered or steerable wheels, which can be pivoted about a steering axis to effect cornering or turning maneuvers of the motor vehicle, whereby different steering angles of the steerable wheels and thus of the motor vehicle can be set. By taking the steering angle into account when setting the basic torque, the basic torque can be adapted as required to the steering angle and thus to cornering or turning maneuvers, so that a particularly advantageous situation-adaptive torque cap can be implemented. As a result, excessive vibrations of the drive train, in particular during turning maneuvers at low speeds, can be avoided.

In order to be able to set the basic torque particularly as required, it is provided in a further embodiment of the invention that a speed of at least one of the wheels, in particular by means of a speed sensor, is detected, the coefficient of friction being determined as a function of the detected speed. As a result, the coefficient of friction can be determined particularly precisely, in particular estimated, so that the basic model can subsequently be adapted or set in a particularly advantageous manner.

Another embodiment is characterized in that the coefficient of friction is determined as a function of a steering angle of the motor vehicle. As a result, the coefficient of friction can be determined particularly precisely so that the basic torque can be set in accordance with the situation. As a result, a particularly high level of driving comfort can be achieved.

In a further embodiment of the invention, at least one acceleration acting on the motor vehicle is detected, in particular by means of at least one acceleration sensor, the coefficient of friction being determined as a function of the detected acceleration. As a result, the coefficient of friction can be determined or estimated particularly precisely, so that the basic torque can subsequently be adapted to the coefficient of friction and thus to the respective driving situation, particularly as required.

In a particularly advantageous embodiment of the invention, the coefficient of friction is determined as a function of a yaw rate of the motor vehicle, whereby the coefficient of friction can be determined or estimated particularly advantageously and precisely.

In a further embodiment of the invention, a friction-locking clutch is used as the clutch, the friction-locking clutch being designed, for example, as a multi-disc clutch. The frictional clutch, which is also referred to as a friction clutch, comprises at least two friction partners, via which torques can be transmitted, in particular depending on the position of the clutch, so that the second wheels can be driven by the drive motor via the friction partners. To set the respective clutch torque described above, the friction partners are pressed together, for example, with a respective contact pressure. The pressing force can be brought about hydraulically, pneumatically or electrically or electromechanically, for example. To adjust the first coupling element, the friction partners are pressed together, for example, by means of a first pressing force, with the friction partners, for example, by means of an opposite one, for realizing the second coupling element the first contact force lower, the second contact force are pressed together. Thus, the friction partners are pressed together in the second position, but less strongly pressed together than in the closed position, so that the clutch is pretensioned in the second position.

By using the frictional clutch, in particular the multi-disc clutch, the clutch can be switched between the second position and the closed position as required, which means that it is possible to switch between two-wheel or front or rear-wheel drive and four-wheel or all-wheel drive as required. In particular, by using a frictional coupling, in particular a multi-disc clutch, it is possible to be able to set the basic torque in a particularly simple and particularly needs-based manner, so that a particularly high level of driving comfort can be achieved.

A second aspect of the invention relates to a motor vehicle designed, for example, as a motor vehicle, in particular a passenger car, with a drive train which is designed to carry out a method according to the invention. Advantages and advantageous configurations of the first aspect of the invention are to be regarded as advantages and advantageous configurations of the second aspect of the invention and vice versa.

Further advantages, features and details of the invention emerge from the following description of a preferred exemplary embodiment and with reference to the drawing. The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures can be used not only in the combination specified, but also in other combinations or on their own, without the scope of the Invention to leave.

The drawing shows in:

Fig. 1

a schematic representation of a drive train of a motor vehicle, a basic torque of a clutch of the drive train being set as a function of a determined coefficient of friction; and

Fig. 2

a block diagram to illustrate a method for operating the clutch.

Identical or functionally identical elements are provided with the same reference symbols in the figures.

Fig. 1 shows a schematic representation of a drive train 10 for a motor vehicle configured, for example, as a motor vehicle, in particular a passenger vehicle. The drive train 10 comprises a clutch, in the present case designed as a multi-plate clutch 12, which is thus designed as a frictional or force-locking clutch. Furthermore, the drive train 10 comprises a drive motor 14, which in the present case is designed as an internal combustion engine or as an internal combustion engine. As an alternative to this, it is conceivable that the drive motor 14 is designed as an electric machine or as an electric motor. In the exemplary embodiment illustrated in the figures, the drive motor 14 comprises a cylinder housing 16, through which combustion chambers in the form of cylinders 18 are formed. Furthermore, the drive motor 14 comprises an in Fig. 1 output shaft which cannot be seen and which is rotatable about an axis of rotation relative to the cylinder housing 16. If the drive motor 14 is designed, for example, as a reciprocating piston engine, the output shaft is designed, for example, as a crankshaft. In the exemplary embodiment illustrated in the figures, the drive motor 14 is designed as a longitudinally installed or longitudinally installed drive motor, the axis of rotation running at least essentially in the longitudinal direction of the vehicle.

The drive train 10 further comprises a transmission 20 which, for example, has an in Fig. 1 includes non-recognizable transmission input shaft and a transmission output shaft 22. The transmission output shaft 22 can be driven, for example, by the transmission input shaft. The transmission input shaft is, for example, via an in Fig. 1 Starting element (not shown) can be driven by the output shaft and thus by the drive motor 14, so that the transmission output shaft 22 can be driven via the transmission input shaft and the starting element by the output shaft and thus by the drive motor 14. The multi-plate clutch 12 can be driven by the transmission output shaft 22 and thus via the transmission output shaft 22, the transmission input shaft and the starting element from the output shaft or from the drive motor 14. For example, in a pulling operation, the drive motor 14 provides driving torques via its output shaft of the motor vehicle ready. The torques can for example be transmitted from the output shaft via the starting element, the transmission input shaft and the transmission output shaft 22 to the multi-disk clutch 12 and introduced into the multi-disk clutch 12, whereby the multi-disk clutch 12 can be driven or is driven.

The drive train 10 comprises a front axle 24, which has front wheels 26 and 28. The front axle 24 is a primary axle and thus a first axle of the drive train 10 or is also referred to as the first axle. The front wheels 26 and 28 are thus first wheels of the drive train 10 or are also referred to as first wheels. The front wheels 26 and 28 can be driven by the output shaft and thus by the drive motor 14, in particular via the starting element. The front axle 24 has a differential 30, which is also referred to as a front axle drive or front axle differential. The front wheels 26 and 28 can be driven by the output shaft via the differential 30 and in particular via the starting element. The differential 30 enables speed compensation between the front wheels 26 and 28, in particular during turning maneuvers or when the motor vehicle is cornering, so that the wheel on the outside of the curve can turn faster than the wheel on the inside of the curve. In other words, the differential 30 allows different speeds of the front wheels 26 and 28.

The drive train 10 further comprises a rear axle 32 which is spaced apart from the primary axle in the vehicle longitudinal direction and is arranged behind the front axle 24 in the vehicle longitudinal direction and which has rear wheels 34 and 36. The rear axle 32 is a secondary axle and thus a second axle of the drive train 10 or is also referred to as a second axle. The rear wheels 34 and 36 are second wheels of the drive train 10 and the rear wheels 34 and 36 are also referred to as second wheels. In the exemplary embodiment illustrated in the figures, the rear axle 32 is thus designed as a hang-on rear axle which can be switched on and off as required. The preceding and following statements can easily be transferred accordingly to such an embodiment in which the rear axle 32 is the primary axle and the front axle 24 is the secondary axle, so that the front axle 24 is then designed as a hang-on front axle and can be switched on and off as required .

The drive train 10 further comprises a shaft in the present case as a cardan shaft 38, the rear wheels 34 and 36 from the transmission output shaft 22 via the cardan shaft 38 and via the multi-plate clutch 12 and thus via the transmission output shaft 22, the transmission input shaft and the starting element from the output shaft and thus from the drive motor 14 can be driven. The rear axle 32 has a second differential 40, which is also referred to as a rear axle differential or rear axle drive. The rear wheels 34 and 36 can be driven by the cardan shaft 38 via the differential 40, the differential 40 allowing different speeds of the rear wheels 34 and 36, particularly during turning maneuvers or when the motor vehicle is cornering. In other words, the differential 40 enables a speed compensation between the rear wheels 34 and 36 so that, for example, when cornering, the wheel on the outside of the curve can rotate faster or at a higher speed than the wheel on the inside of the curve.

The function and structure of the respective differential 30 or 40 are sufficiently known, so that they will only be discussed briefly below using the example of differential 40. As is well known, the differential 40 has a basket 42 on which differential gears 44 are rotatably mounted. Furthermore, the differential 40 comprises output gears 46 which are non-rotatably connected to axle shafts 48 of the rear axle 32. The rear wheels 34 and 36 can be driven via the axle shafts 48. Furthermore, the differential 40 comprises a ring gear 50 which is connected to the basket 42 in a rotationally fixed manner and via which the basket 42 can be driven. The differential gears 44 are in meshing engagement with the output gears 46, the differential gears 44 and the output gears 46 having respective toothings which mesh with one another.

In addition, the drive train 10 comprises a bevel gear 52 which is connected to the cardan shaft 38 in a rotationally fixed manner and which is in meshing engagement with the ring gear 50. This means that the ring gear 50 and the bevel gear 52 have respective toothings which mesh with one another. In this way, for example, the ring gear 50 can be driven by the cardan shaft 38 via the bevel gear 52. If the ring gear 50 is driven by the cardan shaft 38 via the bevel gear 52, the basket 42 is thereby driven by the ring gear 50. As a result, the differential gears 44 and, via them, the output gears 46 are driven, so that the axle shafts 48 and, above that, the rear wheels 34 and 36 are driven.

Out Fig. 2 it can be seen that the multi-plate clutch 12 is used at a front, first separation point T1. In other words, the multi-plate clutch 12 is used to implement the front, first separation point T1.

At a rear, second separation point T2, a form-fitting coupling device, in the present case designed as a claw coupling 54, is used. The claw coupling 54 is adjustable between a coupling position and at least one decoupling position. In the coupling position, the rear wheels 34 and 36 are positively coupled to the cardan shaft 38 via the claw coupling 54, so that in the coupling position torques between the rear wheels 34 and 36 and the cardan shaft 38 can be transmitted via the claw coupling 54 or so that the rear wheels 34 in the coupling position and 36 can be positively driven by the cardan shaft 38 via the claw coupling 54. In the decoupling position, however, the rear wheels 34 and 36 are decoupled from the cardan shaft 38, so that in the uncoupling position of the dog clutch 54 the rear wheels 34 and 36 cannot be driven by the cardan shaft 38 via the dog clutch 54.

The dog clutch 54 is integrated into the differential 40. The dog clutch 54 is arranged in relation to a torque flow from the cardan shaft 38 to the rear wheels 34 and 36 such that the rear wheels 34 and 36 are decoupled from the ring gear 50 in the decoupling position of the dog clutch 54, that is, not via the dog clutch 54 with the ring gear 50 are coupled. The bevel gear 52 and the ring gear 50 form an angular drive designed as a 90-degree angular drive, the rear wheels 34 and 36 being positively coupled to the angular drive via the claw coupling 54 in the coupling position of the claw coupling 54 and thus being drivable by the angular drive via the claw coupling 54 . In the decoupling position of the dog clutch 54, however, the rear wheels 34 and 36 are decoupled from the angular drive, so that the rear wheels 34 and 36 cannot be driven by the angular drive via the dog clutch 54.

The claw coupling 54 comprises, for example, at least one coupling element which is adjustable between the coupling position and the decoupling position. An actuator 56 is provided, by means of which the coupling element can be moved between the closed position and the open position, in particular in a translatory manner.

The cardan shaft 38 and the angular drive are, for example, part of a secondary drive train or form such a secondary drive train, by means of which a four-wheel or all-wheel drive that is particularly economical in terms of space, weight and cost can be created. By using the separation points T1 and T2 and thus the multi-disc clutch 12 and the dog clutch 54, it is possible to switch between two-wheel or front-wheel drive and four-wheel or all-wheel drive, so that the drive train 10 is designed as an all-wheel drive train or as an all-wheel system. The all-wheel or four-wheel drive is a first operating state, the two-wheel or front-wheel drive being a second operating state of the drive train 10 or of the motor vehicle. To implement the first operating state, the multi-disc clutch 12 and the claw clutch 54 are closed, so that the multi-disc clutch 12 is in its closed position and the claw clutch 54 is in its coupling position. If the drive motor 14 is then in its pulling mode, in which the drive motor 14 provides torque via its output shaft, both the front wheels 26 and 28 and the rear wheels 34 and 36 are driven by the output shaft and thus by the drive motor 14. To implement the second operating state, the dog clutch 54 is opened and thereby moved into its decoupling position. In addition, the multi-disc clutch 12 is at least partially or partially opened and thus, for example, moved from its closed position into a second position that is different from the closed position.

The closed position of the multi-disc clutch 12 serves to couple the cardan shaft 38 via the multi-disc clutch 12 with the output shaft and thus the rear axle 32 with the front axle 24 via the multi-disc clutch 12. In other words, in the closed position of the multi-disc clutch 12, the axles are coupled to one another via the multi-disc clutch 12. In the second position of the multi-disc clutch 12, which is different from the closed position, the multi-disc clutch 12 couples the axles less strongly than in the closed position. It is preferably provided that the multi-plate clutch 12 is not completely open in the closed position, but rather closed, but closed to a lesser extent or to a lesser extent than is in the closed position, so that the axes are coupled to one another in the second position of the multi-disk clutch 12 via the multi-disk clutch 12, but are less strongly coupled to one another than in the closed position.

For this purpose, for example, a first clutch torque of the multi-plate clutch 12 is set to achieve the closed position, a second clutch torque of the multi-plate clutch 12 that is lower than the first clutch torque is set to achieve the second position. As a result, the multi-disk clutch 12 can transmit at most a first torque in the closed position, the multi-disk clutch 12 being able to transmit at most a second torque that is lower than the first torque in the second position.

The multi-disc clutch 12 and thus the drive train 10 are preferably overpressed in the closed position of the multi-disc clutch 12 so that there are no differential speeds in the multi-disc clutch 12 or between the axles, i.e. so that there is no slippage in the multi-disc clutch 12 or between the axles . In the second position, however, there may be a slip in the multi-plate clutch 12 or between the axles.

The multi-disk clutch 12 comprises, for example, a plurality of disks, in particular friction disks, which, for example, are arranged one behind the other or one after the other in the axial direction of the multi-disk clutch 12. To implement the closed position and thus the first clutch torque, the disks of the disk clutch 12 are pressed together, in particular in the axial direction of the disk clutch 12, by means of a first contact force. To implement the second position, the disks of the disk clutch 12 are pressed together, in particular in the axial direction of the disk clutch 12, by means of a second contact force which is lower than the first contact force. As a result, the second clutch torque is set as a so-called basic torque of the multi-plate clutch 12, which is pretensioned by the basic torque (second clutch torque). As a result of this pretensioning of the multi-disc clutch 12, the multi-disc clutch 12 can be closed particularly quickly, in particular further, starting from the second position and, for example, moved into the closed position. Such a pretensioning of the multi-disc clutch 12 is therefore advantageous in order to intervene quickly and thus the multi-disc clutch 12 to move particularly quickly from its second position into the closed position, whereby excessive slip between the axles can be avoided.

The drive train 10 further comprises a first speed sensor 58, by means of which a speed of the ring gear 50 can be or is detected. The angular drive has a gear ratio different from 1, for example. Since the ring gear 50 is coupled to the cardan shaft 38 via the bevel gear 52, the speed of the ring gear 50 correlates with the speed of the cardan shaft 38, the speed of the ring gear 50 not necessarily having to correspond to the speed of the cardan shaft 38 in terms of value. Depending on the transmission ratio of the angular drive, however, the speed of the cardan shaft 38 can be calculated from the speed of the ring gear 50 detected by means of the speed sensor 58.

The drive train 10 includes second speed sensors 60a-d, which are also referred to as wheel speed sensors. The respective speeds of the wheels (front wheels 26 and 28 and rear wheels 34 and 36) can be detected by means of the speed sensors 60a-d. In other words, the respective speeds of the front wheels 26 and 28 and the rear wheels 34 and 36 are detected by means of the second speed sensors 60a-d.

The first speed sensor 58 provides, for example, at least one first signal, in particular a first electrical signal, which characterizes the speed detected by means of the speed sensor 58. The respective speed sensor 60a-d provides at least one second signal, in particular at least one second electrical signal, which characterizes the respective speed of the respective wheel detected by means of the respective speed sensor 60a-d. The speed sensors 58 and 60a-d are connected, for example, via respective lines 62 and 64 to an electronic computing device 66 of the drive train 10 and thus of the motor vehicle, the electronic computing device 66 also being referred to as a control device. The respective signals characterizing the respective speeds are transmitted from the speed sensors 58 and 60a-d via the lines 62 and 64 to the control device and received by the control device. The lines 62 and 64 are, for example, components of a data bus system of the motor vehicle, the data bus system also being referred to as a data bus and being designed, for example, as a CAN bus (CAN - Controller Area Network). The respective Transmit signals to the control unit and receive them from the control unit.

The control unit can control the claw clutch 54, in particular the actuator 56, and the multi-disc clutch 12, so that the claw clutch 54 can be adjusted between the coupling position and the decoupling position as a result of such control, or so that the multi-disc clutch 12 between the closed position and the second as a result of such control Position can be adjusted. Since the control unit receives the signals mentioned, it is possible for the control unit to activate and thus operate the dog clutch 54 and the multi-plate clutch 12 as a function of the detected rotational speeds, so that, for example, the dog clutch 54 as a function of at least one of the rotational speeds between the decoupling position and the Coupling position is adjustable or is adjusted. Alternatively or additionally, it is conceivable that the multi-plate clutch 12 is adjusted between the closed position and the second position as a function of at least one of the detected rotational speeds.

Fig. 2 shows a block diagram to illustrate a method for operating the drive train 10. In a first step of the method illustrated by a block 68, at least one coefficient of friction of a roadway on which the motor vehicle is located is determined, in particular by means of the control unit. In other words, if the motor vehicle is supported by the wheels on the roadway, with the motor vehicle driving along the roadway, for example, and thus rolling over the wheels on the roadway, at least one coefficient of friction of the roadway is determined in the first step. The coefficient of friction is also referred to as the road coefficient of friction or road coefficient of friction and is, for example, a particularly dimensionless measure of a frictional force that can be transmitted between the wheels and the roadway and that can be transmitted in relation to a contact force with which the wheels are pressed against the roadway. In other words, what is known as a coefficient of friction adaptation takes place in the first step, within the framework of which the coefficient of friction of the roadway is determined, in particular estimated.

With an im Fig. 2 A second step of the method illustrated by a block 70 results in an adaptive torque cap, within the framework of which, depending on the determined, in particular estimated, coefficient of friction, in particular by means of the control unit, the basic torque of the multi-plate clutch 12 is set, which is biased in the second position by the basic torque.

In Fig. 2 is illustrated by a block 72 that the coefficient of friction is determined as a function of at least one of the detected rotational speeds of the wheels. In particular, it can be provided that the coefficient of friction as a function of the detected speed of the left front wheel 26 and / or as a function of the detected speed of the right front wheel 28 and / or as a function of the detected speed of the left rear wheel 34 and / or as a function of the detected speed of the right rear wheel 36 is determined, in particular estimated, is. Furthermore, in Fig. 2 by a block 74 that, in particular by means of respective acceleration sensors, accelerations acting on the motor vehicle, in particular a longitudinal acceleration acting on the motor vehicle and a transverse acceleration acting on the motor vehicle, are detected, the coefficient of friction depending on the detected ones acting on the motor vehicle Accelerations is determined. In particular, it can be provided that a yaw rate of the motor vehicle is recorded, the coefficient of friction being determined as a function of the yaw rate.

In addition, in Fig. 2 illustrated by a block 76 that, for example by means of a steering angle sensor, a steering angle of the motor vehicle is detected. The coefficient of friction is determined, in particular estimated, as a function of the recorded steering angle. For example, the front wheels 26 and 28 and / or the rear wheels 34 and 36 are designed as steerable wheels which can be pivoted about a steering axis in order to enable the motor vehicle to make turning maneuvers or cornering. By pivoting the respective wheels about the steering axis, respective steering angles of the respective steerable wheels can be set, the steering angle being detected by means of the steering angle sensor.

In addition, in Fig. 2 illustrates a powertrain model by block 78. The drive train model is a computational model stored, for example, in a memory of the control device, on the basis of which a behavior, in particular an expected behavior, of the drive train 10 and thus of the motor vehicle as a whole is calculated by means of the control device.

The motor vehicle comprises at least one control element that can be actuated and thereby moved by the driver of the motor vehicle, by means of which a load of the drive motor 14 and thus the respective torque to be provided by the drive motor 14 via the output shaft can be set. The operating element is designed, for example, as a pedal, which is also referred to as an accelerator pedal and, for example, can be pivoted about a pivot axis into different pedal positions. The driver can press the accelerator pedal with his foot and thereby move it into different pedal positions, the respective pedal position corresponding to a respective torque provided by the drive motor 14. For example, the pedal position of the accelerator pedal is detected by means of a pedal sensor, wherein - as in FIG Fig. 2 is illustrated by a block 80 - the detected pedal position is fed to the drive train model. Thus, for example, the expected behavior is calculated as a function of the detected pedal position. Furthermore - as in Fig. 2 illustrated by a block 82 - the respective torque provided by the drive motor 14, which is also referred to as engine torque, is determined and fed to the drive train model so that the expected behavior is calculated and thus determined, for example, depending on the determined engine torque. The calculated, expected behavior is fed to the coefficient of friction adaptation so that the coefficient of friction is determined as a function of the calculated behavior.

The determined and, for example, estimated coefficient of friction is finally fed to the adaptive torque cap, so that the basic torque of the multi-plate clutch 12 is set as a function of the determined or estimated coefficient of friction.

Out Fig. 1 it can be seen that the secondary drive train is shut down in the second operating state, since the secondary drive train, which is also referred to as the secondary drive train, neither via the multi-plate clutch 12 from the drive motor 14 or from the front wheels 26 and 28, nor via the dog clutch 54 can be driven by the rear wheels 34 and 36. As a result, the drive train 10 is designed as a needs-based all-wheel drive system, it being possible for the secondary drive train to be designed to be particularly economical in terms of weight, installation space and costs. As a result, the energy consumption, in particular the fuel consumption, and thus the CO ₂ emissions of the motor vehicle can be kept particularly low. In other words, to achieve a consumption saving the cardan shaft 38 and the ring gear 50 are at least temporarily shut down, in particular in the second operating state, as a result of which speed-dependent and torque-dependent losses can be kept particularly low. In relation to an entire operating time of the motor vehicle, the second operating state is set significantly more frequently than the first operating state. As a result of the frequent second operating state and the associated lower load spectrum or torque level, the secondary drive train can be designed to be particularly small, as a result of which weight and component costs can be saved. However, the small or filigree dimensioning of the secondary drive train increases its tendency to vibrate, in particular during turning maneuvers, which tendency to vibrate can lead to acoustic impairments.

The front wheels 26 and 28 can be driven by the drive motor 14 when the multi-disc clutch 12 and the dog clutch 54 are closed or are in their closed position or in their coupling position and when the multi-disc clutch 12 and the dog clutch 54 are open or when the multi-disc clutch 12 is in its second position and the dog clutch 54 is in its decoupling position. As a result, the rear wheels 34 and 36 or the rear axle 32 can be switched on or activated as required by the multi-disc clutch 12 and the dog clutch 54 being closed. Furthermore, the rear wheels 34 and 36 or the rear axle 32 can be switched off as required, that is to say deactivated, by moving the multi-plate clutch 12 into its second position and the dog clutch 54 into its decoupling position. By connecting or activating the rear axle 32, the first operating state (four-wheel or all-wheel drive) is set, with the second operating state (two-wheel or front-wheel drive) being set by switching off or deactivating the rear axle 32.

By setting and thus for example by varying the basic torque of the multi-plate clutch 12, excessive vibrations of the drive train 10 and thus excessive noise can be avoided, so that acoustic impairments can be avoided. As a result, a particularly high level of driving comfort can be achieved. In the second position, the multi-plate clutch 12 can be closed as much as possible, however, it should be opened as far as necessary, in order to avoid excessive vibrations of the drive train 10 on the one hand and to advantageously couple the axles to one another on the other hand in order to ensure high traction of the motor vehicle. As a result, a conflict of objectives between the implementation of advantageous, high traction and the avoidance of vibrations can be resolved or at least relaxed.

Overall, it can be seen that the, for example, stationary and steering angle-based basic torque of the multi-plate clutch 12 is increased as a function of the locally estimated coefficient of friction in order to ensure advantageous traction properties. The local estimation of the coefficient of friction takes place, for example, by comparing the model-based expectation of the vehicle behavior with the measured rotational speeds or speeds of the wheels, acceleration, the yaw rate and the steering angle.

By estimating the coefficient of friction, information on the coefficient of friction can be obtained. On the basis of this coefficient of friction information, on the basis of estimated wheel loads and on the basis of a torque of the drive train 10 or in the drive train 10, a calculated vehicle model can be used to proportionally transfer an excess torque applied to the front axle 24, which is configured as the primary axle, to the rear axle 32, which is configured as a secondary axle.

Thus, the engine torque, which is also referred to as drive torque, is primarily deposited on the front axle 24 when the road surface is dry and thus, for example, with a high coefficient of friction, and tensioning of the drive train 10 and thus undesired acoustic feedback can be avoided. With high drive torques or drive forces or with low coefficients of friction, especially on wet roads, the secondary axle is accordingly more involved in order to transfer the drive torque. However, in these situations the risk of tension is significantly lower due to the high wheel slip.

The drive train 10 and the method are based on the idea that at low coefficients of friction, compensatory movements, as a result of which excessive tension in the drive train 10 can be relieved, can take place via the roadway. In the case of high coefficients of friction, such compensatory movements over the roadway cannot take place, or only to a lesser extent. Thus, for example, with high coefficients of friction the base torque reduced on the multi-plate clutch 12 in order to allow compensating movements on the drive train 10. As a result, excessive tension and thus excessive vibrations and undesirable noises can be avoided. Such a reduction in the basic torque can be permitted in the case of high coefficients of friction, since the axles or the wheels have good adhesion to the road surface due to the high coefficient of friction. In the case of low coefficients of friction, the basic torque can be increased in order to couple the axles more closely than a lower basic torque. As a result, excessive slip between the axles can be avoided, so that particularly advantageous traction can be ensured. Since compensatory movements between the axles or in the drive train across the roadway are possible due to the low coefficient of friction, there is no excessive tension and thus no undesirable noise.

Claims (8)

1. Method for operating a clutch (12) of a drive train (10) of a motor vehicle, comprising the clutch (12), a drive motor (14), a primary axis (24) having first wheels (26, 28), which can be driven by the drive motor (14), as a first axis, and a secondary axis (32) having second wheels (34, 36), which can be driven by the drive motor (14) via the clutch (12), as a second axis, wherein the clutch (12) is adjusted between a closed position, in which a first coupling torque of the clutch (12) is set, and at least one second position, which is different from the closed position and in which a lower second coupling torque of the clutch (12), relative to the first coupling torque, is set, comprising the steps:

   - determining at least one coefficient of friction of a roadway on which the motor vehicle is located; and

   - depending on the determined coefficient of friction: adjusting a basic torque of the clutch (12), which is pre-tensioned by the basic torque in the second position;

   characterised in that
   an expected behaviour of the motor vehicle is calculated by means of an electronic computing device (66) of the motor vehicle on the basis of at least one computer model, wherein the coefficient of friction is determined depending on the calculated behaviour.
2. Method according to claim 1,
   characterised in that
   the basic torque is set depending on a steering angle of the motor vehicle.
3. Method according to claim 1 or 2,
   characterised in that
   a speed of at least one of the wheels is detected, wherein the coefficient of friction is determined depending on the detected speed.
4. Method according to any one of the preceding claims,
   characterised in that
   the coefficient of friction is determined depending on a steering angle of the motor vehicle.
5. Method according to any one of the preceding claims,
   characterised in that
   at least one acceleration acting on the motor vehicle is detected, wherein the coefficient of friction is determined depending on the detected acceleration.
6. Method according to any one of the preceding claims,
   characterised in that
   the coefficient of friction is determined depending on a yaw rate of the motor vehicle.
7. Method according to any one of the preceding claims,
   characterised in that
   a friction clutch is used as the clutch (12), in particular a multiple-plate clutch (12).
8. Motor vehicle, comprising a drive train (10), which is designed to perform a method according to any one of the preceding claims.

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